Process for removing oxygen from metal oxides by electrolysis in a fused salt

ABSTRACT

Oxygen is removed from metal or metalloid oxides by electrolysis in a fused salt, to produce elements or alloys, so as to prevent problems of cell contamination and poor power efficiency, particularly in scaled up electro-deoxidation processes. Cell current is controlled not to exceed a selected threshold for substantially all of the electrolysis process. The process may include a first constant current phase and a second constant voltage phase.

[0001] The present invention relates to an improved method for removing oxygen from metal oxides by electrolysis in a fused salt to produce elements or alloys. It is particularly concerned with extraction processes for producing titanium or titanium alloys, especially ones conducted upon a large scale.

BACKGROUND OF THE INVENTION

[0002] Electro-winning of elements from their ores has become well-established over the last two hundred years. In a recent development, a new process has come to light in which a metal oxide is fabricated as an electrode with an electrolytic collector and is cathodically reduced to the metal in the presence of a fused electrolyte and applied direct current voltage. Such an “electro-deoxidation” or “electro-reduction” process, hereinafter generically referred to as an “EDO” process, is described in WO 99/64638 to Fray, and is generally applicable to oxides of metals, metalloids and mixtures of oxides. WO 99/64638, the disclosure of which is incorporated herein by reference, is especially concerned with the extraction of titanium from its oxide, although it teaches that the process may also be applied to decompose electrolytically oxides of U, Mg, Al, Zr, Hf, Nb, Mo and rare earths such as Nd and Sm, as well as oxides of metalloids such as Ge and Si.

[0003] WO 99/64638 teaches that it is important for the potential of the cathode to be maintained and controlled potentiostatically so that only oxygen removal occurs and not the more usual decomposition of the fused salt electrolyte. According to Fray, on the application of a voltage via a power source, a current will not start to flow until balancing reactions occur at both the anode and the cathode. At the cathode there are two possible reactions, the discharge of the cation from the salt or the removal of oxygen. The latter reaction occurs at a more positive potential than the discharge of the metal cation and, therefore, will occur first. However, for the reaction to proceed, it is necessary for the oxygen to diffuse to the surface of the oxide and, depending on the temperature, this can be a slow process. For best results it is, therefore, important that the reaction is carried out at a suitably elevated temperature, and that the cell potential is controlled, to prevent the potential from rising and chlorine evolution from the electrolyte occurring as a competing reaction to the oxygen being removed from the oxide.

[0004] It will, of course, be understood that the explanation presented in WO 99/64638 on how electro-deoxidation occurs is merely a theory, and is not intended to impose any limit on the protection conferred to the present invention.

[0005] The benefit from electro-reduction by an EDO process is that high yields can be achieved with less energy compared to other extraction processes.

[0006] In WO 99/64638, the extractions in the detailed Examples are conducted upon a laboratory scale, employing a carbon anode and conducted at a constant voltage. If attempts are made to scale up such a process, for example for industrial use, however, a number of problems arise that were not encountered by Fray, including poor power efficiency and problems of short circuiting and electrolyte contamination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention will now be described in more detail, with reference to the accompanying drawings in which:

[0008]FIG. 1 shows a typical current profile for the electro-deoxidation of a metal, metalloid or mixed oxide using an EDO process at constant voltage; and,

[0009]FIG. 2 shows a preferred current profile for the electro-deoxidation of a metal, metalloid or mixed oxide using the process according to the present invention.

DESCRIPTION OF THE INVENTION

[0010] In accordance with the present invention, a method is provided of extracting a metal, metalloid or alloy from a metal oxide, metalloid oxide or mixture of oxides of alloying elements, by conducting electrolysis in a cell comprising a carbon anode and a fused electrolyte under conditions so as to remove oxygen from the oxide or oxides, wherein the current is controlled so that it does not exceed a selected threshold throughout substantially all of the electrolysis process.

[0011] In order for the selected threshold not to be exceeded for substantially all of the extraction process, the current needs to be limited for at least a first portion of the process, typically for up to the first quarter or first third of the overall reaction time, where it would otherwise be too high. The active control may merely comprise setting the voltage to a suitably low value, preferably having regard to the resulting anodic current density, and may optionally include periodic testing at a higher, more desirable operating voltage to check if the threshold is still being exceeded.

[0012] The present invention relates, in particular, to the production of titanium from titanium dioxide by an EDO process; thus the metal or alloy preferably comprises titanium. However, the method is applicable to all elements that can be extracted using an EDO process and such elements include, for example, V, U, Mg, Al, Zr, Hf, Nb, Mo and rare earths such as Nd and Sm, as well as metalloids such as Ge and Si.

[0013] The present invention is of especial application in scaled up processes, for example where more than 0.5 kg, preferably 1 kg, and usually more than 10 kg of oxide undergoes electro-deoxidation in a single cell, or especially in industrial scale processes where oxide batches exceed 30 kg/cell or even 40 kg/cell and more than 20 kg of metal is extracted per cell per day. This is because larger scale cells have been found to be increasingly more susceptible to the problems outlined above.

[0014] We have found that when the EDO process, as taught by WO 99/64638, is scaled up using a carbon anode and a calcium chloride electrolyte, deposits are formed on the surface of the electrolyte. These deposits build up into an undesirable crust, which crust presents several problems, for example decreased power efficiency, increased processing time, possible short circuits and electrolyte contamination. The deposits appear to come from the anode. Anode degradation and weight losses of up to 10% are expected in electro-deoxidation processes, especially with lower density, industrial quality carbon anodes, because the released oxygen forms combustion products (e.g. CO, CO₂). However, it would appear that some carbon loss cannot be accounted for as combustion products.

[0015] It is common practice to use carbon anodes in high temperature electrolysis cells comprising fused salts. Moreover, carbon anodes are known to be capable of withstanding high current densities without a problem, for example, current densities of the order of 10,000 A/m² are commonly applied to the anodes of Hall cells.

[0016] We have surprisingly found that the formation of the crust can be prevented by controlling the current in the electrolysis cell such that the current does not exceed a selected threshold value, which value appears to be related to a critical anodic current density, above which anode degradation occurs. The current generated and, hence, the anode current density, is directly related to the size of the oxide sample. Thus, the laboratory scale Examples of WO 99/64638 did not encounter the problem of crust formation. Limitation of the current is effective in preventing the build-up of deposits on the surface of the electrolyte, irrespective of the magnitude of the applied voltage, provided, of course, that the voltage remains within the required range for electro-deoxidation by an EDO process.

[0017] Whenever an EDO process is conducted by the usual constant voltage method, the cell current is not constant. Typically, under the voltage control method of running, there is a large current peak near the start of the process, which peak is believed by the present inventors to be caused by a combination of a large applied potential and some easily removed oxygen (the latter having variable binding energies). A typical current profile is depicted in FIG. 1 for constant voltage. As explained above, typical peak currents for scaled up titanium extraction processes correspond to anode current densities well below those commonly withstood by carbon anodes in other electrolytic processes. However, when an electro-deoxidation process is employed upon a large scale, such carbon anodes appear to undergo some additional form of chemical attack.

[0018] In one scaled up titanium extraction process run using lkg titanium oxide as the starting material, the overall electrolysis time, at a constant voltage of 3.2V, was approximately 50 hours. Referring to FIG. 1, the peak current, which typically occurs after approximately 30 minutes, had a magnitude of around 200A. A second peak, or current plateau, occurred approximately 10-15 hours from the start of the electrolysis run. Using the process of the present invention, a current threshold for the electro-deoxidation of 1 kg titanium dioxide was then selected at approximately 70A, using a 3.5 kg, cylindrical carbon anode arranged concentrically about the cathode.

[0019] WO 99/64638 refers briefly to the possibility of controlling current in order to address a charge imbalance problem created in the first few minutes of operation, but that is a different problem to the one addressed by the present invention.

[0020] The present process may be conducted substantially as a two-stage process comprising a first current controlled phase, wherein the current is limited and held below an identified threshold, but preferably kept as high as possible, and a second voltage limited phase wherein the voltage is held at the maximum allowable for electro-deoxidation by an EDO process.

[0021] A typical current profile for electro-deoxidation according to the present invention is illustrated in FIG. 2. In the figure, ‘I’ represents the cell current in Amps and ‘t’ represents the time in hours. The dotted line delimits the two electro-deoxidation stages.

[0022] During the first stage, the cell current is prevented from exceeding the selected threshold for substantially all of that part of the process when it would otherwise do so. Additionally, the voltage is capped to prevent decomposition of the electrolyte. The second stage commences when a transition point is reached at which, even at maximum voltage, the current is no longer able to exceed the threshold. This transition point, represented by A on FIG. 2, occurs toward the end of the deoxidation process, typically after three quarters of the oxygen has been removed. During the second stage, the voltage is still capped, so as to prevent chlorine evolution.

[0023] A consequence of varying the operating voltage such that the cell current is limited is that an EDO process takes longer to complete. It will generally be desirable for the electro-deoxidation reaction to proceed as fast as possible, so preferably the current is maintained as close as possible to the selected threshold value. Advantageously, the current is maintained within 20%, and ideally within 10%, of the selected threshold until, even at maximum voltage, the current cannot exceed the threshold. In other words, the current is maintained as close as possible to the threshold until the transition point mentioned above is reached.

[0024] Usually, the two-stage process will be conducted using automated control means that steadily increase the voltage in the first stage, and then maintain constant high voltage once the above-mentioned transition point has been reached.

[0025] One preferred method of limiting the cell current to the required value is to vary the applied cell voltage within the limits required for electro-deoxidation. Preferably the current is prevented from exceeding the selected threshold by use of a control means. More preferably, the control means comprises a current measuring device and a variable voltage supply. Alternatively, the control means may comprise a fixed voltage supply and a variable resistor, which resistor limits the current drawn by the cell. Other control means will suggest themselves to the skilled addressee. Automated control means under computer control will usually be preferred, and may be programmed to follow various operating modes.

[0026] Analysis of the crust has shown that it comprises substantial amounts of carbonaceous material, for example carbon, calcium carbide and calcium carbonate, which material is formed from anode degradation. We postulate that the anode degrades by a combined chemical and mechanical etching mechanism, the chemical mechanism playing a more significant role in the total degradation in an EDO process than in other electrolytic processes. It will be understood that this is merely a theory on how the deposit is reduced and is not intended to impose any limitation on the claimed invention.

[0027] Depending on the cell design, the selected threshold may lie at any point on the profile illustrated in FIG. 1, and may, for example, be at a current threshold below the second plateau. Usually, the current threshold is selected such that no substantial visible deposits are formed on the surface of the electrolyte under selected atmospheric conditions. More preferably, the current threshold is selected such that no short circuiting is caused by surface deposits.

[0028] We have found that the degree of crust formation is dependent on the surface area of the anode and, hence, the actual operating current density. Therefore, the current threshold may alternatively be selected such that the current density at the anode is prevented from exceeding a critical limit. Preferably, the current threshold is selected such that the current density at the anode does not exceed 200 A/m², although in some cell arrangements, for example ones employing reduced pressure, current densities of up to 500 or even 600 A/m² may still be advantageous. The anode size and configuration may also be selected having regard to the required current density.

[0029] An additional benefit of the present invention is that the peak power consumption of the electrolytic cell is reduced compared with a cell operating at constant voltage. This is an important factor in scaling up the process, in that it permits the use of less expensive power supplies.

[0030] According to WO 99/64638, the electrolyte must consist of salts which are preferably more stable than the equivalent salts of the metal which is being refined and, ideally, the salt should be as stable as possible to remove the oxygen to as low as concentration as possible. The electrolyte may be a halide salt, preferably a chloride salt, and the cation may be selected from barium, calcium, caesium, lithium, strontium and yttrium. Mixtures of salts may also be used.

[0031] Calcium chloride, or a mixed salt containing calcium chloride, is inexpensive and commonly employed, providing a wide temperature of operation without excessive vaporisation. For the electro-deoxidation of metal oxides in electrolytes containing calcium chloride, at approximately 1000° C., the applied operating voltage should lie in the range 1.4 to 3.4V, and more preferably in the range 1.7 to 3.2V.

[0032] We have found that limiting the cell current is an effective method of preventing the formation of a carbon crust during a scaled-up EDO process. However, in an important aspect we have found that controlling the atmosphere can assist in preventing crust formation, as well as significantly reducing the overall reaction time. In one preferred aspect of the invention, the atmosphere is maintained at a reduced pressure for at least part of the electrolysis process, for example, at least in the first stage identified above, using, for example, a means such as a vacuum pump. Preferably the pressure is below 100 mBar. More preferably the pressure is in the range 1 to 10 mBar. Operating the electrolysis cell at reduced pressure causes vigorous agitation of the electrolyte surface, which assists in breaking down and oxidising the crust. Typically, the crust can be removed in 5 to 10 minutes.

[0033] The atmosphere will often comprise an inert gas, such as argon, and may optionally include small amounts of oxygen. This may encourage the oxidation and removal of carbonaceous deposits.

[0034] The following Examples illustrate the invention:

EXAMPLE 1

[0035] A cathode was constructed by threading 1 kg of porous titanium dioxide tiles onto a stainless steel rod. The cathode was mounted in an electrolytic cell containing calcium chloride electrolyte at approximately 1000° C. and a 3.5 kg carbon anode.

[0036] The titanium dioxide was electrolysed for 50 minutes at 2.2V, at the end of which time the current had increased to 88A and deposits were starting to form on the surface of the electrolyte. The voltage was reduced to 2.0V, which resulted in the current decreasing to 70A and no further formation of deposits. The run was continued for a further 17 hours at a maximum current of 70A, during which time no crust was observed on the electrolyte.

[0037] This Example shows that crust formation can be prevented by operating the cell at a current slightly below that at which substantial deposits are observed to form.

EXAMPLE 2

[0038] Porous tiles were formed by sintering a mixture of titanium, aluminium and vanadium oxides, and a cathode was subsequently constructed by threading 439 g of the tiles onto a stainless steel rod. The cathode was mounted in an electrolytic cell containing calcium chloride electrolyte at approximately 950° C. The cylindrical anode consisted of industrial quality carbon (density 1.8 g/cm³) with a surface area of 1650 cm².

[0039] The mixed oxide was electrolysed for 77½ hours at a fixed current of 20A (current density 121 A/m²), using a constant current transformer to apply the cell voltage. A voltage cap of 3.2V was set up, but during the run the voltage did not, in fact, exceed 2.6V. Therefore, the electro-deoxidation process was not completed and had not yet reached the second stage described above and illustrated in FIG. 2. At the end of the run, the cathode had been partially reduced to a Ti/Al/V alloy with approximately 10 wt% remaining oxygen.

[0040] During the run, no carbon crust or detrimental surface deposits formed, so this Example shows that controlling the current, and hence the anode current density, such that it does not exceed a selected threshold is an effective method of preventing crust formation.

EXAMPLE 3

[0041] A cathode was constructed by threading 1 kg of porous titanium dioxide tiles onto a stainless steel rod. The cathode was mounted in an electrolytic cell containing calcium chloride electrolyte at approximately 1000° C. and a carbon anode.

[0042] The titanium dioxide was electrolysed for 1½ hours at 2.0V, at an average current of 49A, after which time a significant crust was observed on the surface of the electrolyte. A vacuum was applied to the cell for 2 minutes at a pressure of 10 mBar and the crust was observed to decrease, but not disappear. The pressure was increased again to atmospheric pressure and electrolysis continued, with the crust subsequently reforming.

[0043] This Example shows that reduced pressure can be used temporarily to decrease the amount of surface crust. 

1. A method of extracting a metal, metalloid or alloy from a metal oxide, metalloid oxide or mixture of oxides of alloying elements, by conducting electrolysis in a cell comprising a carbon anode and a fused electrolyte under conditions so as to remove oxygen from the oxide or oxides, wherein the current is controlled so that it does not exceed a selected threshold throughout substantially all of the electrolysis process.
 2. A method according to claim 1 wherein the metal or alloy comprises titanium.
 3. A method according to claim 1 wherein the electrolyte comprises calcium chloride.
 4. A method according to claim 1 wherein more than 0.5 kg of oxide undergoes electro-deoxidation.
 5. A method according to claim 1 wherein the current threshold is selected such that no substantial visible deposits are formed on the surface of the electrolyte under selected atmospheric conditions.
 6. A method according to claim 1 wherein the current threshold is selected such that the current density at the anode does not exceed 500 A/m².
 7. A method according to claim 1 wherein the current is maintained within 20%, and ideally within 10%, of the selected threshold until, even at maximum voltage, the current cannot exceed the threshold.
 8. A method according to claim 1 wherein the current is prevented from exceeding the selected threshold by use of a control means.
 9. A method according to claim 8 wherein the control means comprises a current measuring device and a variable voltage supply.
 10. A method according to claim 1 wherein the atmosphere is maintained at reduced pressure for at least part of the electrolysis process.
 11. A method according to claim 10 wherein the pressure is below 100 mBar.
 12. A method according to claim 11 wherein the pressure is in the range 1 to 10 mBar. 